1. Field of the Invention
The invention relates to a bicycle wheel having tension spokes and to a method for manufacturing such a wheel. More particularly, in a non-limiting embodiment, the rim and spokes of the wheel are made from a composite material.
2. Description of Background and Relevant Information
As is known, a wheel with tension spokes includes a peripheral rim having a channel for receiving a tire, typically a pneumatic tire, as well as a central hub and spokes extending between the rim and the hub.
Spokes are usually distributed in two sets, the spokes of each set connecting the rim to respective ones of the transverse ends of the hub. The spokes are tensioned so that the various components of the wheel, i.e., the rim, the hub and the spokes, form a rigid assembly.
Important properties of a wheel include its weight, rigidity, and fatigue strength.
The weight essentially depends upon the materials used. As a general rule, the rim is made of an aluminum alloy or other lightweight material, and the spokes are made of stainless steel or an aluminum alloy.
Wheels are also known in which the rim and/or spokes are made of a composite material. Indeed, a composite material has certain advantages over commonly used metals. In particular, a composite material has a high modulus of elasticity, 125,000 MPa (Megapascals) compared to 72,000 MPa for an aluminum alloy 7075, and a low density, 1.55 kg/m3 compared to 2.8 kg/m3 for the aluminum alloy 7075, and 7.8 kg/m3 for stainless steel.
Furthermore, a composite material has a fatigue strength that is markedly greater than that of metals. When the wheel is rotationally driven, the spokes are subject to repeated tensile and release stress cycles; the rim is also locally subject to compressive and tensile stress cycles. For a spoke made of metal, these stresses can, over time, cause cracks leading to ruptures. A spoke made of a composite material has a much more stable behavior with respect to fatigue strength.
The rigidity of a wheel is evaluated by its resistance to deformation. Usually, one distinguishes the front rigidity, i.e., the resistance to deformation in reaction to an impact or a frontal force, and the lateral rigidity subsequent to an impact or a lateral force. One seeks particularly to stiffen the wheel laterally.
The rigidity of a wheel mainly depends upon the spoke tension that exists between the rim and the hub. The spoke tension must be sufficiently high to prevent any spoke from becoming loose during normal operation of the wheel. If a spoke were to loosen momentarily due to an external load, it would be as if the spoke did not exist. The rim is no longer supported by the loose spoke and the rim becomes deformed under the effect of the forces to which it is subjected due to the load and to the tension exerted by the other spokes. Thus, the more tensioned the spokes, the farther the threshold beyond which the tension of a spoke is cancelled by the action of an external load, and the more rigid the wheel.
The tension of a spoke must however be considered from a relative standpoint. By definition, the rim is in static balance with respect to the hub under the effect of the tension of all of the spokes. Under these static conditions, the tension of a spoke is reactively taken up by the other spokes depending upon their position in relation to the spoke considered. Dynamically, a variation in the tension of a spoke is carried over to the other spokes in the form of an increase or a decrease in their own tension.
The most common manner of tensioning spokes involves associating each of the spokes with a screw/nipple system with which the effective length of the spoke can be adjusted.
Such a tensioning device is advantageous because it makes it possible to adjust the overall tension in the sets of spokes and to locally adjust the geometry of the rim, i.e., to true the rim, by local action on the tension of one or several given spokes. Geometrically, the parameters of a rim that can be adjusted include lateral runout (deformation of the rim along an axial direction), centering (relative position of the axis of revolution of the rim and the axis of rotation of the hub), or radial runout (deformation of the rim along a radial direction).
The overall tension of the spokes is adjusted by more or less tensioning all of the spokes, taking into account their mechanical properties. This determines the rigidity of the wheel. A flaw in the geometry of the wheel can furthermore be corrected by locally adjusting the tension of one or several spokes by means, for example, of a screw/nipple system integrated in the area of the fastening ends of the spoke. Adjusting the geometry of a rim can be a time-consuming and expensive process, as it requires each spoke to be individually adjusted.
Such an adjusting device is known for metallic spokes, but also for spokes made of a composite material. In this regard, the following patent documents can be cited: FR 2 586 378; U.S. Pat. No. 6,036,281; EP 872 357; EP 1 044 827; and FR 2 792 251. The device for tensioning the spokes can be found at the connection between the spoke and the rim, between the spoke and the hub, or along the spoke itself.
Such a device yields good results when associated with each spoke. However, this significantly increases the weight and inertia of the wheel. Moreover, the screw/nipple connection of the tensioning device creates weakened zones where the transmission of forces and the fatigue strength are not controlled as well as in the body of the spoke itself. In the case of composite spokes, the use of a screw/nipple adjusting device, which is necessarily metallic and, therefore, sensitive to the phenomena of fatigue rupture, diminishes the fatigue strength of the spoke/adjusting device assembly and, therefore, limits the advantage of using a composite spoke. These screw/nipple adjusting devices also constitute a weight increase, also reducing the advantage of using composite spokes.
Another method for tensioning the spokes involves spacing the spoke sets apart in the area of the hub. U.S. Pat. No. 5,104,199 discloses a spacing system in the area of the hub, with split washers that are housed in grooves of the hub body for maintaining the spacing between the two spoke sets. By spacing the spoke sets apart in the area of the hub, all of the spokes in the two sets are tensioned simultaneously. The spokes themselves do not have their own tensioning mechanisms. Such methods are used in connection with composite wheels. These methods are efficient because the resulting wheel is essentially a monolithic structure, and no unnecessary adjusting metal element increases the mass of the wheel or weakens the wheel. As a result, the wheel is lighter in weight and has less inertia. However, once the spokes are tensioned, the geometry of the rim cannot be modified. Under these conditions, this geometry is dependent upon the precision with which the manufacture and assembly of the various elements were carried out. The fact that the wheel does not include any final adjusting mechanism may result in wasting a substantial quantity of material and in considerably increasing the production cost. Moreover, the monolithic structure of the wheel does not allow repairs, since the wheel, along with the hub, must be entirely replaced in the case of a problem.
In view of this prior art, there is a need for a wheel with tension spokes, and for a wheel having an improved system for tensioning spokes, as well as a method for manufacturing same, which is simplified while remaining efficient with respect to the mechanical properties and geometric parameters of the rim. There is also a need for a wheel whose useful life is increased, and whose sensitivity to the phenomena of fatigue rupture is reduced. There is also the need for having the capability of making adjustable and lightweight spoke wheels made from a composite material.
Finally, there is the need to increase the speed of adjusting the geometric characteristics of a wheel.